When an automatic transmission of a vehicle is set to a forward drive position and the vehicle is stopped, driving force of an idling engine is transmitted through a torque converter to the transmission and wheels. As a result, creeping occurs. Such creeping leads to lower mileage when the vehicle is stopped. Therefore, in a state in which a brake pedal is stepped on to operate the brake and an accelerator is substantially fully closed so that the vehicle is stopped in forward drive position, neutral control takes place, in which the transmission is set to an almost neutral state with the forward drive position being maintained, in order to attain better mileage. In the neutral control, an engaging element that is engaged at the start of the vehicle is released, to bring the transmission to the substantially neutral state. When returning conditions such as releasing of the brake are satisfied, a return control takes place, in which the released engaging element is engaged again.
By way of example, Japanese Patent Laying-Open No. 2005-42742 discloses a starting controller that enables reliable return from the neutral control. The starting controller is for a vehicle mounting an automatic transmission having an engaging element that is engaged when the vehicle starts. If the vehicle stops in the forward drive position while predetermined conditions of vehicle state are satisfied, the neutral control is executed and the engaging element is released, and if separately determined conditions are satisfied, a return control from the neutral control is executed. The starting controller includes means for detecting engine speed, and calculating means for calculating engaging hydraulic pressure of engaging element on return from the neutral control based on engine speed fluctuation.
According to the starting controller disclosed in the laid-open application, a constant standby pressure is calculated in consideration of engine speed fluctuation. As a result, a vehicle starting controller can be provided, which enables reliable return from the neutral control in accordance with the engine state, on return from the neutral control.
It is possible, however, that necessary hydraulic pressure of engaging element differ at every return from the neutral state, because of variations in torque ratio change in a torque converter or engine torque. Therefore, in the return control from the neutral control, necessary engaging hydraulic pressure of engaging element would not be obtained with high accuracy if the control much depends on feed forward control. If the hydraulic pressure supplied to the engaging element on the return from neutral control is too high or too low, abrupt engagement at the start of return control or abrupt engagement at the end of return control occurs, possibly resulting in a shock.
In the laid-open application mentioned above, simply the constant standby pressure is changed in accordance with the engine speed fluctuation and it much depends on feed forward control. Therefore, such a problem cannot be solved.
Further, in the return control, relative rotation speeds on the input shaft side and output shaft side of engaging element are decreased for transition to the engaged state. Therefore, even when the hydraulic pressure of engaging element is adjusted so that turbine speed comes close to a target value, it is sometimes impossible to obtain necessary engaging hydraulic pressure of engaging element with high accuracy, because of variations in engine torque or torque ratio change.